The invention presented relates to gas and/or fluid flow measurement techniques making use of thermal effects. Convection plays a role here: a moving medium entrains heat (energy) by way of its own heat capacity.
A known sensor, in which use is made of said property, is the anemometer. As a rule, the embodiment of such measuring device consists of one or more objects which are heated by a specific dissipated power and in which the flow affects the resulting temperature of said object. Said resulting temperature is a measure of the flow. Said embodiment and method are referred to as xe2x80x9cConstant Power Anemometryxe2x80x9d or simply CPA method. Also, said object may be kept at a constant temperature difference with respect to a reference temperature, and it may be measured which power to be dissipated is required for that purpose. Said embodiment and method are referred to as xe2x80x9cConstant Temperature Anemometryxe2x80x9d or simply CTA method.
Apart from this, the measuring devices based on thermal effects may be (mechanically or fluidally) broken down into three types:
(1) devices in which the object is entirely enclosed by the medium, as disclosed in U.S. Pat. No. 4,651,564 [R1, 1987] and in [R2, 1993];
(2) devices in which the object is not on all sides in touch with the medium, as extensively discussed in [R3, 1995]; and
(3) devices in which the object, contrary to (1), encloses the medium {the medium flows through a tubular devicexe2x80x94the object), as disclosed in U.S. Pat. No. 5,036,701 [R4, 1990].
The known xe2x80x9chot-wire anemometryxe2x80x9d measuring devices are of type (1), [R5, 1995].
An example of said type also is the method and device as disclosed in [R1, 1993], in which the power dissipated in the object is kept constant (CPA), and in which the temperature (distribution) around the object referred to earlier is then considered.
The invention relates to the control of the temperature of objects which are in thermal interaction with the environment. To support the specification of the invention, the so-called heat balance of an object is first gone into.
The behaviour of an object (having on it e.g., a temperature sensor and/or a heating element) in a thermal interaction with the environment may be studied by considering the heat balance of said object.
The heat balance refers to the circumstance that the increase of the heat Q (energy dimension) stored in the object equals the heat transmitted to it (Pin) or generated within the object (Pgen), minus the heat dissipated (Pout) and absorbed within the object (Pabs):                                           ⅆ                          Q              obj                                            ⅆ            t                          =                              P                          i              ⁢                              xe2x80x83                            ⁢              n                                +                      P            gen                    -                      P            out                    -                      P                          ab              ⁢                              xe2x80x83                            ⁢              s                                                          (        F1        )            
For the sake of clarity of the further specification, it is assumed that the internal heat absorption (e.g., such as the one by way of an internal chemical reaction or a phase transition) equals zero (Pabe=0).
The heat capacity of the object (Cobj) determines the relationship between the stored heat (energy) and the temperature of said object (Tobj):                               T          obj                =                              Q            obj                                C            obj                                              (        F2        )            
The heat balance of an object then becomes:                                                         ⅆ                              Q                obj                                                    ⅆ              t                                =                                    P              gen                        +                          P                              i                ⁢                                  xe2x80x83                                ⁢                n                                      -                          P              out                                      ⁢                  
                ⁢                              ⅆ                          T              obj                                            ⅆ            t                          =                                            P              gen                        +                          P              inout                                            C            obj                                              (        F3        )            
with Pinout=Pinxe2x88x92Pout (F3)
Said latter formula or differential equation for Tobj may be written in the form of an integral:                               T          obj                =                              1                          C              obj                                ⁢                      ∫                                          (                                                      P                    gen                                    +                                      P                    inout                                                  )                            ⁢                              ⅆ                t                                                                        (        F4        )            
An object is in equilibrium with the environment if Tobj=constant (no longer depending on time). Therefore, the following applies to said object:
Pgen+Pinout=0, or Pgen=xe2x88x92Pinout.
From this, it is obvious that, for a situation in which the temperature of the object (Tobj) is constant there exists a balance between the internally generated power Pgen and the incoming and outgoing power Pinout.
The temperature of an object which is in thermal interaction with the environment having a flowing medium, in equilibrium has a temperature which may be expressed as follows:                                           T            obj                    -                      T            m                          =                              P            gen                                A            +                          B              ⁢                                                "LeftBracketingBar"                  v                  "RightBracketingBar"                                                                                        (        F5        )            
where Tobj is the object temperature, Tm is the medium and environmental temperature, Pgen is the heat generated within the object, A is a constant representing the heat conduction from the object to the environment, B is a constant representing the influence of the convection of the medium on the object temperature, and v is the flow rate of the medium.
Here, four items of interest are distinguished:
1 For a given and fixed Pgen, A, B and v, the (absolute) temperature of the object is directly proportional to the temperature Tm of the medium.
2 For a given and fixed A, B and v, the temperature difference Tobjxe2x88x92Tm is proportional to the internally generated heat Pgen: Tobjxe2x88x92Tm=constant.Pgen 
3 The proportionality constant is referred to as the xe2x80x9cflow sensitivityxe2x80x9d of the object: Tobjxe2x88x92Tm=G(v).Pgen 
4 Only the size of the flow affects the temperature and not the sign (positive or negative direction).